The present invention relates in general to web accumulators for accumulating and discharging a reserve portion of a continuous web passing through the accumulator to enable continuous operation of processing stations on either or both sides of the accumulator when the speed of the web moving through the processing stations temporarily varies between the two stations. More particularly, the present invention relates to a control arrangement for belt-powered web accumulators that limits torque disturbances between the input and output rollers of such accumulators.
A typical web accumulator consists of sets of fixed and movable web rollers with the web path passing around these rollers so that the length of accumulated web increases when the moveable rollers move away from the fixed rollers and decreases when the moveable rollers move toward the fixed rollers. In order to accumulate web, the velocity of the web flowing into the accumulator must exceed the velocity of the web flowing out of the accumulator. Similarly, to discharge web, the velocity of the web flowing out of the accumulator must exceed the velocity of the web flowing into the accumulator. The input and output rollers of accumulators may be powered by servomotors or drive shafts, while the remaining rollers in the accumulator are idler-rollers that are rotated by the web moving over the rollers.
Since idler rollers have inertia and a coefficient of drag associated with their rotary motion, a force must be imparted by the web to accelerate, maintain radial velocity, and decelerate each idler roller. Therefore, each idler roller in the accumulator induces undesired tension variations in the web. Because web tension is proportional to web strain, any tension variation also creates a strain variation.
For processes that are to deliver fixed amounts of relaxed web per unit time wherein the web is elastic and exhibits elastic behavior at least for strain values, it is common to define and elastic modulus E that describes the relationship between strain, ε, in the direction of web flow, and tension, T, per unit width of web. For a given width of web, a web modulus, Ew, is defined which describes the relationship between web tension, T, and web strain, ε, in the direction of web-flow. This relationship is: T=εEw. For many materials, Ew, and therefore T, vary even within a particular lot of material. Such variations are no problem provided strain remains within the elastic region of the web; and, therefore, the primary objective for processes that deliver fixed amounts of relaxed web per unit time is to maintain target strain, rather than target tension, within acceptable limits.
In processes where strain variations need to be kept to a minimum and for weak webs in general, the size of the accumulator is limited by the number of idler rollers that can be turned by the web without the web being over-strained. Singh, U.S. Pat. No. 4,009,814, which is incorporated herein by reference, solves the strain problem resulting from idler rollers by introducing a chain or belt that is wrapped around sprockets or pulleys associated with the rollers in the accumulator so that each roller in the accumulator is powered by the same power sources that drive input and output rollers, respectively. Further, the rate of web accumulation or discharge is controlled by the difference in velocity between the input roller and the output roller. Herein, the Singh type of driven accumulator will be referred to as a belt-powered accumulator.
It is known to use servo-drives to drive belt-powered accumulators. However, unless the load inertia reflected onto each servomotor is negligible compared to the motor inertia, a substantial torque coupling can exist between the input and output roller servo drives. This torque coupling induces undesired speed variations on the input roller and the output roller when the opposing torque between the input roller and the output roller changes.
There is thus a need to provide a control arrangement for driven belt accumulators that limits torque disturbances between the input and output rollers of the accumulators.